Korkeusero on suuri.

Because it is a Finnish compound noun. Finnish makes compounds very often, and they are usually written as one word, not two.

So korkeusero is one unit, just like English can have words such as height difference, even though English usually writes them separately. In Finnish, the equivalent is normally written together.

Writing korkeus ero would be incorrect here.

It is made of:

• korkeus = height
• ero = difference

So grammatically, it is a compound meaning height difference.

A useful thing to notice is that Finnish often builds compounds out of nouns, not adjectives. That is why you get korkeus (height) rather than korkea (high).

Because korkea is an adjective meaning high, while korkeus is a noun meaning height.

In this sentence, the idea is not high-difference, but height-difference. So Finnish uses the noun korkeus as the first part of the compound:

• korkea = high
• korkeus = height
• korkeusero = height difference

This noun-based compounding is very common in Finnish.

On is the 3rd person singular present tense of olla, which means to be.

So:

• olen = I am
• olet = you are
• on = he/she/it is

In this sentence, on works exactly like English is.

It is in the nominative singular.

Here it is the subject of the sentence, so the basic dictionary form is used:

• korkeusero = nominative singular

In a simple sentence like this, the subject often appears in the nominative.

Because suuri is a predicate adjective after olla (to be).

In Korkeusero on suuri, the structure is:

• korkeusero = subject
• on = verb
• suuri = predicate adjective

With a singular subject like this, the adjective appears in the basic nominative singular form:

• suuri = large / great

So suuri is not modifying a noun directly inside a noun phrase; it is describing the subject through the verb on.

Because Finnish has no articles.

That means Finnish does not normally use separate words corresponding to English a, an, or the. Context tells you whether something is definite or indefinite.

So korkeusero can mean:

• a height difference
• the height difference

depending on the situation.

The most neutral word order here is:

• Korkeusero on suuri.

That is the normal subject + verb + complement order.

Finnish word order is fairly flexible, but changes usually add emphasis, contrast, or a more marked style. So this sentence is the plain, natural way to say it.

Yes, often you can.

• suuri = large, great
• iso = big, large

So Korkeusero on iso is possible in everyday Finnish.

However, suuri can sound a bit more neutral or standard in many written or formal contexts, while iso is often a little more everyday and conversational.

If you are talking about more than one height difference, you would say:

• Korkeuserot ovat suuria.

Notice the changes:

• korkeusero → korkeuserot
• on → ovat
• suuri → suuria

This is a useful pattern to learn, because Finnish often changes the predicate adjective when the subject becomes plural.

Roughly:

• korkeusero → KOR-keu-se-ro
• on → on
• suuri → SUU-ri

A few helpful points:

• The main stress in Finnish is usually on the first syllable.
• uu in suuri is a long vowel.
• Finnish spelling is very regular, so words are usually pronounced much as they are written.

So the sentence has the main stresses on KOR- and SUU-.